Miscellaneous Science Questions

[Eebster the Great]

Also make sure you always hit the same part of the mug. The pitch will vary with the position relative to the handle.

[Sizik]

Also make sure you always hit the same part of the mug. The pitch will vary with the position relative to the handle.

As demonstrated here

[Heimhenge]

Okay, explain this to me, all you lay physicists (or, um, even any professional physicists, you're cool too):

1. Make a nice steaming mug of hot cocoa. Use a reasonably sturdy mug.
2. Put a spoon in the glass and begin steadily tapping at the bottom. The sound of the tapping will rise in pitch.
3. Do it less steadily. The pitch will not cease to rise. There will be no noticeable correlation between the rate or regularity of the tapping and the rate of change in pitch.
4. Walk away and leave the entire thing alone for a few minutes.
5. Come back and start tapping again. IT WILL CONTINUE AT THE SAME PITCH IT LEFT OFF AT.

Why is this? If the pitch rises due to, say, energy imparted to the glass or the beverage, it should decay over time, which would mean that a slower rate of tapping would result in a smaller rate of change in pitch, and walking away for a few minutes should cause the pitch to drop noticeably. If it's some kind of resonance in the chamber, it should simply emphasize frequencies close to the resonant frequency of the glass and de-emphasize others. The heck's going on here?

That seems like it'd have to be a subjective effect. The resonant frequency should only depend on the size/shape of the resonator, and the speed of sound in the liquid. Don't see how tapping rate could have an effect.

I've noticed something similar for ages but never got around to actually investigating the effect. When I stir my instant coffee into a cup of microwaved hot water, the clinking of the spoon against the porcelain mug starts at a higher frequency and drops to a lower frequency as I stir. Not much ... maybe a half-note. I always attributed it to the expansion of the mug as it absorbed heat which would lower the resonant frequency. But I still wonder if maybe it might have something to do with the coffee dissolving and changing the density of the liquid?

[commodorejohn]

Hmm, that's an interesting comparison. But what I'm noticing (if, again, it's not purely perceptual/brain weirdness) is on the order of a good octave or more total change over time.

[Eebster the Great]

It does seem like this would be pretty easy to resolve with a cup of hot chocolate, a spoon, and a microphone.

[commodorejohn]

It does. A guy should probably get around to that at some point.

[SuicideJunkie]

It does seem like this would be pretty easy to resolve with a cup of hot chocolate, a spoon, and a microphone.

It would probably be worth adding a metronome to consistently tap the spoon as well.

[p1t1o]

I've noticed this.
I thought it was fairly obvious that as the teaspoon cools, its resonant frequency changes due to minute changes in size and flexibility as it contracts.
A shorter, stiffer spoon rings at a higher frequency.
I presume the effect described in point #5 is subjective and would be eliminated on examination.

[jewish_scientist]

Yesterday a student in my environmental anthropology class claimed that global climate change can cause volcanoes to erupt more frequently. When I asked for an explanation, they just muttered something about tectonic plates under the ocean and CO₂. There is no way that this can be correct. The mass of the Earth's mantle and crust is several magnitudes greater than that of the atmosphere, so how can anything involving the atmosphere be non-trivial?

[gmalivuk]

I don't think it would affect suboceanic plates or be a direct consequence of CO₂ concentrations, but the existence of a connection isn't totally far-fetched.

If some type of tectonic activity is already near a tipping point, then even small changes in pressure (such as air pressure in a storm) or mass distribution (such as from rain, flooding, and resulting mudslides) could potentially trigger something.

No one's arguing that climate change impacts the overall direction of plate tectonics, but individual events don't depend on anything as large as the entire crust or mantle.

[p1t1o]

There have been seismic events linked to things like dams filling (and the 3 gorges dam in china measurably affected the rate of Earth's rotation), so its not super crazy, especially if the sea level rises land-based ice melts significantly. Its not about excess weight, its about it being re-distributed. Its not going to cause a 2012[movie]-like shift of the continents or anything stupid though, and I highly doubt it is going to have global effects like "volcanoes will erupt more frequently".

https://en.wikipedia.org/wiki/Induced_seismicity

[Eebster the Great]

If this is just tipping things like quakes and eruptions over the edge, wouldn't they happen eventually anyway? I'm not sure I understand how they could make them more frequent in the long run.

[eSOANEM]

unrelated to the above, just a random fleeting thought:

if we started selectively breeding ballerinae, roughly how many generations would it take before they become ungulates (due to the selective pressure of dancing en pointe)?

[Liri]

unrelated to the above, just a random fleeting thought:

if we started selectively breeding ballerinae, roughly how many generations would it take before they become ungulates (due to the selective pressure of dancing en pointe)?

Don't forget to work in the diet of course grasses.

Within 100 generations is "rapid evolution", so we're probably looking at at least that, even with the artificial selection. Only a couple millennia to achieve perfection.

[jewish_scientist]

If a species reproduces asexually, then what gender are members of that species?

[Zohar]

Gender is a concept that relates to the culture of a species, not reproduction. Sex is probably what you're looking for here. Why would there be different sexes in these species at all?

[gmalivuk]

For larger animals (such as reptiles that reproduce through parthogenesis) scientists generally refer to all of them as females, because they lay eggs or give birth.

[doogly]

If a species reproduces asexually, then what gender are members of that species?

It's polite to just ask them.

[Eebster the Great]

There are also species that can reproduce both asexually or sexually (this is especially common in plants). In some cases, all members of those species are hermaphrodites. In other cases, some but not all members could be hermaphroditic, or none could be. Some plants and animals even undergo sex-switching.

[jewish_scientist]

There are actually some snakes that can reproduce sexually and asexually even though they are not hermaphrodites. In the case of asexual reproduction, the female lays an egg containing a clone.

[Zohar]

There are actually some snakes that can reproduce sexually and asexually even though they are not hermaphrodites. In the case of asexual reproduction, the female lays an egg containing a clone.

If you already know the answer to your own question, may I ask why you posed it in the first place?

[Liri]

I had been meaning to ask for a while whether there was a theoretical "maximum temperature" and decided to look it up before asking here. Lo and behold, there is! And it's actually pretty interesting.

[jewish_scientist]

You should look into negative temperatures, which are all actually hotter than positive temperatures.

[eSOANEM]

This is a lot less weird when you realise that Thermodynamics is generally a lot nicer when you formulate it in terms of β=1/(k_BT) (this does result in heat flowing from low β to high which is clearly wrong so instead of the usual normalisation scheme of setting k_B=1 we ought to go for k_B=-1 although I am aware that this is an unpopular opinion)

[RyvenZ]

New guy hoping to glean wisdom from other XKCD fans; I have a question I am unable to find the answer for.

Napoleon Dynamite's Uncle Rico character wagers he could "throw a football over them mountains" and I wanted some help building a formula that I can use to figure out how impossible it would be to do such a feat. I took physics a long time ago and had forgotten most of the physical world stuff and we never did go over air resistance, which cannot be ignored if this is to be answered correctly.

Is this something that we can simply get to a formula requiring only angle and release velocity?

Knowns:
* An NFL football weighs 14-16 oz. Most physics seems to be in metric, so let's say 450 g as a near middle estimate
* The density of air is 1.2 kg/m3.
* The coefficient of drag for the football is 0.05 to 0.14

I've tried asking this on reddit a while back, and it got no traction so was never answered.

Thanks in advance!

[Eebster the Great]

The most important parameters are the distance to them mountains and the height of them mountains. The others have only minor significance. You also can't really do anything with just density of air and coefficient of drag, but at the end of the day, drag isn't worth worrying about anyway, because you can't throw a football nearly that far in a vacuum anyway.

[DavidSh]

... because you can't throw a football nearly that far in a vacuum anyway.

That depends on another parameter, the acceleration of gravity. On a suitably sized asteroid, you might be able to throw a football that far. If your suit is sufficiently flexible.

[Xanthir]

But a football cannon *could* launch it that far in a vacuum, theoretically. (Well, assuming that the football can stand up to the acceleration of the cannon. But imagine an arbitrarily-long football-railgun that can bring it to the desired velocity at an appropriately low acceleration.)

But when you do take drag into account, you'll find that there is absolutely no way to do it; footballs are fairly low mass, and while they slide air around themselves pretty reasonably at the speed/distances that normal throwing takes place, the speed you'd need to hit to counteract drag over the distance of a mountain-transit would absolutely destroy the football upon contact with said air.

[Eebster the Great]

Fair enough. The circumference at the equator of an NFL football is about 22 inches apparently, so if we model it as a right circular cone, its cross-sectional area is about 250 cm². I'll accept the drag coefficient values, and I'll give Uncle Rico the benefit of the doubt and assume he has a perfect spiral, so 0.05. The force of drag is F = ½ρAC_dv², where ρ=1.3 kg/m³ is the density of air, A=250 cm²=0.025 m² is the cross-sectional area of the football, C_d=0.05, and v is the speed of the football. That simplifies to F = (0.00081 Kg/m) v², or if the football's mass is 420 g, an acceleration of a = (0.0020 m^-1) v².

The problem is still that we don't know how far away or tall them mountains are, so we can't solve for v. Let's say v=1 km/s. Then a=2 km/s², or 200 g. But I don't know if that's a reasonable v.

[Copper Bezel]

I've noticed something similar for ages but never got around to actually investigating the effect. When I stir my instant coffee into a cup of microwaved hot water, the clinking of the spoon against the porcelain mug starts at a higher frequency and drops to a lower frequency as I stir. Not much ... maybe a half-note. I always attributed it to the expansion of the mug as it absorbed heat which would lower the resonant frequency. But I still wonder if maybe it might have something to do with the coffee dissolving and changing the density of the liquid?

I know this is an old post, but they're describing the hot chocolate effect, right?

[gmalivuk]

Fair enough. The circumference at the equator of an NFL football is about 22 inches apparently, so if we model it as a right circular cone, its cross-sectional area is about 250 cm². I'll accept the drag coefficient values, and I'll give Uncle Rico the benefit of the doubt and assume he has a perfect spiral, so 0.05. The force of drag is F = ½ρAC_dv², where ρ=1.3 kg/m³ is the density of air, A=250 cm²=0.025 m² is the cross-sectional area of the football, C_d=0.05, and v is the speed of the football. That simplifies to F = (0.00081 Kg/m) v², or if the football's mass is 420 g, an acceleration of a = (0.0020 m^-1) v².

The problem is still that we don't know how far away or tall them mountains are, so we can't solve for v. Let's say v=1 km/s. Then a=2 km/s², or 200 g. But I don't know if that's a reasonable v.

You can figure out how high and far that v would get you and then decide whether it's reasonable.

For example, while we don't know the exact distance or height, we can at least guess that "them mountains" are far enough away that Rico isn't already at the base of them (else they'd be "these mountains" or "this mountain"). We can also guess that they're at least on the order of 1km high, else they'd hardly warrant being called mountains.

So, can 1000m/s get you a kilometer up at a horizontal distance of more than 1km?